Microwave gas tubes



(kt-'23, 1956 R. A. HAGAN MICROWAVE GAS TUBES Filed May 15 1951 lll/lll] R Y o6 E Mm ,wm

MV n .and this h lweak ionization of a limited volume a lglow discharge.

ydenum or e ed, and' Vincreased for nited States Patent O 2,768,320 h y MICROWAVE GAS TUBES Richard Arthur Hagan, Reading, Massr,1 assignor tov Syl- Vania Electric Products Inc., a corporation of Massachusetts Application May 15, 1951', Serial No. 226,483

7 claims. (013131-198) y The present invention relates to electrical discharge devices and in particular to attenuator and switch tubes of the character used in microwave circuits. The invention is applicable both to coaxial and waveguide forms of circuit structure, but f'or convenience this description is addressed t'o the waveguide form; lts application to the other is readily apparent.

Waveguide attenuators and duplex'ers or transmit-receive tubes, commonly termed TR tubes, and antitransmit-receive tubes, usually referred to as ATR tubes, commonly depend for their performance on a sustained glow-discharge current through a gas in the region `f a further pair of electrodes which effectively constitute a spark discharge gap. For example, a pair of conical electrodes extending from opposite sides fof a waveguide can be' used to initiate an intense ionized discharge when a signal of high energy level is beingt'ransmitted, a high value of oscillating voltage being impressed between these electrodes at the time. Commonly, such conical electrodes are interconnected by armetallic path found in the wall of the waveguide itself. The Yelectrodes are usually associated with an iris whose effect is to resonate with the capacitance of the gap electrodes and thereby to establish a peak voltage difference between the electrodes in the presence of high frequency energy. k

A class of tub'esoffthis character contains a' gas fill-,- any of a wide variety of compositions and under various degrees of pressure. In orderto rendert-ransmit-receive tubes and attenuator tubes of this class more effective, it is desirable to maintain a limited volume of gasjcon'- tinually weakly ionized in the region of the discharge gap while the vtube ismin operatiom during low-level sig.- nal conditions. For this purpose an additional 'electrode is frequentlymounted `closeto the discharge electrode, either concentrically within one of the electrodes jor disposed externally thereofradjacent thel gap, this electrode being termed a keep-alive or keep-alive electrode. A directvcurrent is passed continuously or intermittently depending upon the service between the gap electrodes additional discharge electrode for sustaining of the gas, that is,

The volume of weakly ionized gas that effective to 'speed the response to high-level energy bursts of a very low order and therefore only a smallamount of current need be supplied to the keep-alive electrode, lof the .orderof -50 microamper'es. However, it is sometimes necessary to increase theV volume of gas to, be ionized as 'in the .case of attenuator tubes aridlrine certain broad-band 4transmit-receive and anti-transmit-receiye tubes ,in which the resonant structures are of low-Q design. With the yconventional electrode construction any attempt to in- ,crease the ionization by increasing the current density is destructive 'to 'the keep-'alive despite the 'common practice of forming these of refractory metalsuch asy molybyen tungsten'that'isknown asan electrqnemitter. Current the order of 1000 microamperes'is needfthe'si'ze `of the 'electrodecannot be appreciably 'correspondingly reducing the current den- 4sity the electrode.

objectfof the vpresent invention lresides in the provision of a new form of -keep=alive 'electrode suitable 'for maintaining gas in more intensely ionized state than has trode l146 by the waveguide.

heretofore been practicable and also in the provision of microwave gaseous discharge devices capable of sustain-Y ing increased-keep-alive current. More broadly, the presl ent invention aims at providing microwave gaseous discharge devices of improved operating characteristics. i

In order' that the ionized discharge that occurs during high level bursts of energy may be quicklyl extinguished, the gaseous lill of this type of tube commonly has heretofore included amixture of an inert gas, preferalA bly a noble gas whose ions conibine only with each other after ionization,v together with a quenching gas. Water vapor has been found particularly effective as the quenching gas. The present invention in more specific aspect provides a gaseous discharge tube and a keep-alive elec` trode for such tube which is compatible with such dis-Y charge gaseous fill.

In the illustrative disclosure below a broad band combined attenuator and transmit-receive tube is described having afill of an argon-water vapor mixture. The keepalive electrode in such tube is a composite structure including a core of aluminum and a refractory glass-sealing metal shell of molybdenum.

The illustrative embodiment is shown in the accompanying drawings, wherein:

Figure l is an enlarged longitudinal cross-section of a keep-alive electrode embodying features of the invention,

Figure 2 is the longitudinal cross-section of a combined attenuator and transmit-receive tube containing plural keep-alive electrodes of the yform shown in Figure 1, andl Fig. 3 is a cross-sectional view illustrating more clearly the configuration of the iris plates 20.

Referring now tothe drawing, a rectangular length of waveguide 10 is shown having flanges 12 by means of which the device can be mounted in a waveguide system, and each end of waveguide 10 is provided with a resonant window 14 having a glass or ceramic center portion and a metal bezel or frame portion of conventional construction. Opposite pairs of conical discharge electrodes 16 and 18 afford ya series of discharge gaps, electrodes 18 being of closed deformable construction for critical adjustmentv by engagement from the exterior. Associated with each discharge Vgap is a laterally disposed pair of plates 20,` having a configuration shown more clearly in Fig. 3, which afford lan iris having a center opening for gap electrodes 16 and 18 and which, with those gap electrodes, constitutes 'a resonant aperture through which microwave energy of low level can be transmitted from one window 14 to the other. The discharge gaps and irises are separated from each other along the waveguide by critical spacing, commonly being a distance equal to an effective quarter wave length at the center frequency of the band of frequencies for which the device is to be effective.

Electrodes 16 are hollow in the advantageous form of construction `shown and each contains concentrically a keep-alive electrode 22 whose inner end is limited to a spacing of a few thousandths of an inch from the opening in electrode 16, both radially and endwise. Electrode 16 is accordingly closer to the other discharge gap electrode 1 8 than keep-alive electrode 22 is to discharge gap electrode 1S. Only ay small-end area of the keep-alive is exposed'to the gas, in the region of the gap.

In operation, a direct current is .passed between electrode 22 andelectrode 16, andsome of this passes also to electrode 18 which is metallicall-y connected lto elecy Several hundred bolts must normallyv 'be applied in an example, in a* combined transmit-receive and attenuatorn tube having a fill of argon and water vapor in the 'ratio of 3 to l0, respectively, for the `total pressure of v25 lmillimeters of mercury. A her.

metio glass "seal 2d insulates the vr'rie'tallic portion ofthe 3 keep-alive electrode as'sembly from the waveguide structure.

When the required potential is applied between the keep-alive electrodes and the waveguide structure, a small volume of ionized gas is locally established in each discharge gap. This is effective to attenuate low level signals that may be impressed at one end of the waveguide for transmission therethrough, variably as a function of the degree of ionization. In the event of high level signal bursts, the ionized volumes of gas promote abrupt formation of intensity ionized discharge at any or all of the gaps and this intense discharge is effective for switching the waveguide transmission path from one in which low level signals are transmitted through the structure to one in which high level signals are almost entirely reected.

The multiplicity of discharge gaps enhances the broad band characteristics of the structure. With such construction the resonance of the gap structures are of reduced eect in establishing a high microwave voltage difference at the discharge gaps even when high level signal bursts occur, Low-Q window and iris designs also reduce the microwave voltage level that is built up in each gas-and-iris assembly. With such low Q structure it is desirable to increase the intensity of ionization at each discharge gap in order to promote the desired attenuation of relatively low level signals and to promote fast response to high level signal bursts. It is accordingly desirable that the keep-alive electrode should be capable of carrying sufficiently intense ionizing currents without, however, deleteriously affecting the gaseous till.

It is not practical simply to increase the size of the keep-alive for increased current-carrying capacity, because of microwave design requirements. I have discovered that with a mixture of noble gas and water vapor, sufficiently high currents can be produced by forming a tip 26 on the keep-alive electrode with a shell or tube 28a of molybdenum or other refractory metal capable of sealing to glass. A glass sheath or bead 30 is formed continuously on the outer surface of tube or shell 28a and its integral lead-in 28, extending through seal 24 to terminal 28. Bead 30 serves to prevent a discharge occui-ring between the keep-alive electrode and the metallic waveguide structure at all points except in the immediate region of the discharge gap between electrodes 16 and 18.

Initially. lead 28 is formed with a longitudinal center bore to constitute shell 28a, and with a transverse bore 28h. and is then beaded with glass. An aluminum pin is inserted into the longitudinal center bore of shell 28a, of suicient length to project outside the bore, and with the structure inverted, as compared to the position in Figure l, the aluminum is melted by radio frequency induction in an inert or reducing atmosphere so as to become keyed in the transverse bore 281: in shell 28a. The end surface of the glass and metal keep-alive electrode structure is ground flat. The glass beaded lead is hermetically sealed to a threaded metal tube 32 that is ultimately sealed in a sleeve 34 and joined hermetically to the waveguide structure. When assembled, the keep-alive is critically adiusted to proper position in relation to discharge gap electrodes 16 and 18 for concentricity and axial positioning.

The aluminum tip or core is effective to provide sufficient ionizing current for eifective attenuation and broad band transmit-receive tube application, yet is of no noticeable influence on the composition of the gaseous mixture contained in the tube, and is not attacked so as to increase the critical spacing of the tip from the gap electrodes.

It will be recognized that the foregoing description of an illustrative embodiment is susceptible of varied moditication and rearrangement and that its features can be variously applied by those skilled in the art; and in consequence is appropriate that the appended claims be accorded a broad latitude of interpretation such as is consistent with the spirit and scope of the invention.

What is claimed is:

1. An electric discharge device having an envelope containing a mixture of a quenching gas and an ionizable gas and a pair of electrodes adapted to sustain a glow discharge therebetween, one of said electrodes constituting a cathode and having a film of glass sealed on the exterior thereof except in a confined discharge region, and having an internal core of aluminum exposed to the gas in l that region.

2. A microwave electric discharge tube including a metal-walled envelope dening a transmission path for microwave energy and having means for admitting microwave energy to said path, an ionizable gas in said envelope, opposed elements defining a spark gap therein, and a keep-alive electrode adjacent the gap for maintaining in weakly ionized condition a restricted volume of gas, said keep-alive electrode including a metal rod, an insulating layer fused thereon and covering all of the surface thereof exposed to the gas except in the region of said gap, and a core of low-work-function material in said keepalive at the exposed portion thereof.

3. A combined attenuator and microwave switch tube, including an envelope defining a wave transmission path and enclosing a pair of opposed electrodes constituting a spark gap and an ionizable gas, one of said electrodes being hollow and having a small aperture at the spark gap, a keep-alive electrode within said hollow electrode having a protective insulating sheath covering the lateral surface thereof and exposing an end surface thereof in the immediate vicinity of the discharge gap, said electrode being of a refractory metal and extending through an insulating hermetic seal, and an aluminum insert in said refractory metal exposed to said gas in the immediate vicinity of the discharge gap.

4. A microwave electric discharge device having an envelope containing a mixture of a noble gas and water vapor, opposed elements deninga spark gap therein, and a keep-alive electrode adjacent the spark gap for maintaining in weakly ionized condition a restricted volume of gas, said keep-alive electrode having a glass seal through the envelope wall and a lateral lm of protective insulation, and an exposed tip portion of aluminum.

5. An electric discharge tube including an envelope containing an ionizable gas, a pair of opposed electrodes defining a spark gap therein, and an additional electrode in close proximity to the discharge gap for establishing a glow discharge, said additional electrode embodying a metal shell, and external insulating sheath thereon, and a core filling said Shell of a low-work-unction material.

6. An electric discharge device having an envelope containing an ionizable gas, a pair of opposed elements dening a spark gap therein, and an additional electrode constituting a cathode for making a glow discharge during operation, said additional electrode having a lm of glass covering the exterior thereof except in a confined discharge region, and having a. core tilling the interior thereof, said core being of aluminum and being exposed to the gas in said discharge region.

7. A keep-alive electrode comprising an elongated metallic member having a tapered end with concentric and transverse intercommunicating bores formed in said end, and aluminum substantially filling said bores.

References Cited in the le of this patent UNITED STATES PATENTS 682,694 Hewitt Sept. 17, 1901 1,363,636 Brace Dec. 28, 1920 2,039,167 Hopkins Apr. 28, 1936 2,419,903 McCarthy Apr. 29, 1947 2,433,813 Hilliard Dec. 30, 1947 2,491,971 Hall et al. Dec. 20, 1949 2,524,268 McCarthy Oct. 3, 1950 

